How do i...create a singleton C# form

Question

1.00/5 (2 votes)

See more:

I have a C# windows Form application that creates multiple instances of the same window, when I double click the shortcut, but I only want one single instance of the window.
Can someone please tell me how to accomplish this?
Thanks.

What I have tried:

I tried what was described in this video:
The Singleton Pattern - YouTube[^]

Posted 7-Jan-17 3:02am

Member 10850253

Updated 7-Jan-17 9:06am

Add a Solution

3 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Wendelius · Answer 1 · 2017-01-07T03:08:00

Solution 1

You could use a singleton pattern to ensure that only one instance exists, but I think that it would be easier just to loop through Application.OpenForms Property (System.Windows.Forms)[^].

Loop through the collection, check if the form exists and if it does, activate it, if it doesn't, create it.

[EDIT]
An example loop to iterate through the forms

C#

bool formOpen = false;

foreach (Form form in Application.OpenForms) {
   if (form.Name == "Form1") {
      formOpen = true;
      break;
   }
}
if (formOpen) {
   MessageBox.Show("Form is already open");
} else {
   MessageBox.Show("Form is not open");
}

Posted 7-Jan-17 3:08am

Wendelius

Updated 7-Jan-17 4:07am

v3

Comments

Member 10850253 7-Jan-17 9:37am

I tried comparing the Application.OpenForms.Count==0, and if that was true, then I ran the application, but that doesn't work, and I was wondering if you could tell me where do I need to use this property?

Wendelius 7-Jan-17 9:44am

I'm sorry but I don't fully understand. Are the forms in different applications?

In other words are you trying to prevent an application from running if it's already executing or are you trying to prevent a form from opening twice inside a single application?

Member 10850253 7-Jan-17 9:53am

yes

Wendelius 7-Jan-17 9:55am

Which one is yes? Only one application or only one form inside an application?

Member 10850253 7-Jan-17 9:56am

one form in an application

Wendelius 7-Jan-17 10:07am

Have a look at the modified answer.

Member 10850253 7-Jan-17 11:05am

I added a solution so you can view the code, can you please help?

Wendelius 7-Jan-17 12:18pm

The form isn't found because you have placed the code in the beginning of the program where no forms are yet created.

The main method is typically run only once, when the application starts. at this point only one form is created using the Run method of Application class. So there is no way two similar forms can be created at this point inside a single application.

Can you explain the situation in more detail?

Member 10850253 7-Jan-17 13:58pm

I don't know where to place that code inside my form, to prevent it from creating 2 instances. I know there should be something like new form() somewhere, but I don't know.

Wendelius 7-Jan-17 14:02pm

But if you don't have code

new BatteryMonitorForm()

anywhere else but in Application.Run(new BatteryMonitorForm()); then you won't have two forms of that class...

What is the situation where you have 2 BatteryMonitorForm forms?

Member 10850253 7-Jan-17 15:14pm

What I don't want to happen is for the app to open 2 instances of the app, if I double click the app's shortcut twice.

Wendelius 7-Jan-17 15:25pm

So you want to prevent a second instance of the application from running. This requires a mutex. You need to examine if a mutex is already created and if not you reserve it. Each instance of your application does this check so only the first one is allowed to run.

There's a good post on the subject, have a look at c# - What is the correct way to create a single-instance application? - Stack Overflow[^]

Maciej Los · Answer 2 · 2017-01-07T09:06:00

Solution 3

I'd suggest to read past answers[^].

Posted 7-Jan-17 9:06am

Maciej Los

Member 10850253 · Answer 3 · 2017-01-07T05:05:00

I did this in program.cs

C#

public static bool bf = false;
        static void Main()
        {
            foreach (Form form in Application.OpenForms)
            {
                if (form.Name == "BatteryMonitorForm")
                {
                    bf = true;
                    break;
                }
            }
            if (!bf)
            {
                //Application.EnableVisualStyles();

                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new BatteryMonitorForm());
            }
            else { }
            
            }
            }

And it still doesn't work, I don't know what to do.
Can you please help?
Thanks.